The research aims of this study are to relate the relatively new areas of cytogenetics and human behavior genetics to problems in normal human development and psychopathology. We are investigating the nature of the relationship which exists between sex chromosome aneuploidies in males (XXY,XYY) and subsequent personality development. Infants with these chromosome lesions are being located through a mass chromosomal survey of consecutive male newborns in a Boston obstetrical hospital. These neonates are being followed in a long-term longitudinal study in an attempt to isolate early temperamental characteristics that might be significant with respect to future behavioral abnormalities. Investigators have increasingly emphasized the frequency with which severe psychopathology of a sexual or aggressive nature is found in males with sex chromosome aneuploidies. This evidence has often been obtained by surveying selected groups of individuals who demonstrated a particular personality deviation and finding an increased incidence of a specific chromosomal aneuploidy among them. However, since the actual incidence of the various sex chromosome aneuploidies among the general population is still not fully defined, the appearance of both a sex chromosome aberration and behavioral abnormality in a given individual might represent a chance occurrence. The ongoing mass chromosomal survey of newborn males will help establish the prevalence of these imbalance states among the general male population. Should the evidence continue to suggest that a special relationship appears to exist between sex aneuploidies (XXY,XYY) and psychopathology of a sexual or aggressive nature, the long-term longitudinal studies of the developing personalities of these infants might lead to a further understanding of the mechanisms involved in this relationship.